The invention relates to a semiconductor device for producing or amplifying electromagnetic radiation with a semiconductor body comprising a highly doped semiconductor substrate of a first conductivity type, which has a substantially flat surface provided with at least one strip-shaped raised portion, a blocking semiconductor region of the second opposite conductivity type located on the substrate on either side of the raised portion, a passive layer-shaped semiconductor region of the first conductivity type forming a pn junction with the blocking region, a thin active semiconductor layer disposed thereon, a passive semiconductor layer of the second conductivity type located on the active layer, the active layer having a higher refractive index for the said radiation than the adjoining passive semiconductor regions, and contact layers which are connected in an electrically-conducting manner to the passive layer of the second conductivity type and to the substrate.
The invention further relates to a method of manufacturing the semiconductor device.
A semiconductor device having the structure described above is known from the article of H. Blauvelt et al in Applied Physics Letters, Vol. 41, No. 10, 15 November 1982, p. 903-905. The device described therein is a semiconductor laser in which the strip-shaped region within the active layer is defined at least for a considerable part by a buried blocking region of a conductivity type opposite to that of the adjoining semiconductor material. This has given advantages with respect to double hetero-junction (DH) lasers in which the strip-shaped active region is defined at the exterior of the semiconductor crystal, for example by means of a slot in an oxide layer, or by means of a proton bombardment. For example, inter alia the threshold current is generally lower than that of DH lasers having a more conventional "stripe" structure.
A great disadvantage of the laser structure according to the aforementioned publication in Applied Physics Letters is, however, that in this case the said passive layer-shaped region of the first conductivity type cannot easily be made both very thin and of uniform thickness, whereas in order to prevent an excessively large current spreading a very thin passive region is often desirable. This is due to the fact that as a result of the process used the blocking layer in the proximity of the raised portion of the substrate varies very strongly in thickness, which results in that the passive layer of the first conductivity type grown onto it has a substantially planar upper surface in the proximity of the mesa-shaped raised portion of the substrate only when it is comparatively thick. Such a planar upper surface is desirable in order to obtain a likewise substantially planar active layer, which is often to be preferred with a view to, e.g., the stability of the laser characteristics, the symmetry of the light distribution and a regular growth. A further disadvantage of the known device is that it cannot be manufactured in a continuous growing process, because the growing process has to be interrupted for a "melt back" step.
This is connected with the fact that in a semiconductor device according to the aforementioned publication the width of the "mesa" or raised substrate portion has to be very small because the said melt back process is very critical and may lead to irregularities in the case of wider mesa structures. Such very narrow mesa structures lead to high local current densities and a higher series resistance.